A key aspect of sedimentary rocks is that most are layered or bedded due to the gradual accumulation of material. Depending on the type of material deposited three major classes of sedimentary rocks are recognized, clastic, chemical and organic. Clastic rocks such as sandstone consist of crystals and fragments of older rocks that have undergone weathering processes. Transport of the weathered material under a variety of conditions in a wide range of possible environments eventually results in the accumulation of the sediment. Some rocks such as limestone may be produced by chemical processes as well as biological processes (e.g. coral reefs). The extensive iron ore deposits are also likely to be chemically precipitated minerals. In contrast, an accumulation of abundant plant material in favourable (swampy) environments eventually results in organic rocks, such as coal.
Clastic sedimentary rocks can be classified according to grain size, grain shape and degree of sorting. The finest grained rocks consist almost entirely of clays (i.e. claystones) whereas at the coarse end there are conglomerates. The shape of the individual grains can range from highly angular (as in tuffaceous ejecta) to well rounded (recycled and aeolian sandstones). The degree of sorting reflects the transportation processes and provides a depositional history.
Chemical rocks have a wide range of textures and structures, e.g. oolites and stylolites, respectively, and chemically precipitated calcareous minerals may comprise substantial components of most other sedimentary rock types. The calcareous cements of arenaceous rocks (sandstone) is a prime example.
During sedimentation numerous sedimentary structures may form. All provide some insight into the localized hydraulic conditions affecting the recently deposited sediment. Additionally, structures may form after the sediment is deposited from crustal disturbances, uneven loading, entrapment of fluid, and biologic activity.
Once the process of sediment accumulation has commenced (generally in a basinal environment) the sediment gradually undergoes change. Among the first processes of change is dewatering and compaction due to loading. This is the beginning of diagenesis which gradually converts unconsolidated sediment into rock. With increasing thickness of the sediment pile thermal effects gradually and progressively modify the existing mineralogy resulting in recrystallization, dissolution, redistribution and the formation of new minerals. At this point there commences an overlap with metamorphism.